Although the advent of recombinant DNA technology has resulted in a rapidly expanding list of peptide-based drugs, a major drawback of peptide-based therapy has acutely hampered realization of the full potential of this field: in general, peptide-based drugs cannot be orally administered in effective doses, since they are rapidly degraded by enzymes in the gastrointestinal tract before they can reach the bloodstream. Unless the polypeptide of interest can be altered to make it relatively resistant to such enzymes, the only practical method of delivering the drug is likely to be a parenteral route, such as by intravenous, intramuscular, or subcutaneous injection. Administration by other parenteral routes (e.g., by absorption across nasal, buccal or rectal membranes, or via the lung) has met with limited success.